This invention relates to electronic apparatus for capturing and digitizing x-ray patterns. More particularly, it relates to apparatus for imaging the x-ray pattern resulting from the diffraction of x-rays by crystals.
X-ray diffraction patterns are useful in the analysis of molecular structures, such as protein and virus molecules. Protein and virus crystallography imposes stringent requirements on x-ray detectors, particularly where the x-ray source is high flux synchrotron radiation that enables an experiment to be done rapidly. Furthermore, an important and developing protein-crystallography field is time-resolved crystallography using synchrotrons. Monitoring a time-dependent reaction in a crystal can elucidate the time-dependent molecular changes that occur in a chemical reaction. High time resolution speed is often critical to such monitoring.
Film has traditionally been used in crystallographic analysis but now competes with storage phosphor (SP) imaging plates. Film has poor dynamic range and non-optimal sensitivity because of low stopping power, particularly for higher crystallographic x-ray energies. SP imaging plates have much greater dynamic range than film and are much more convenient to use because an SP laser readout leads directly to a digitized image. However, in spite of the greater SP stopping power, SP sensitivity is relatively as low as film because of photon losses in the readout system. See E. F. Eikenberry, et al, "X-ray Detectors: Comparison of Film, Storage Phosphors and CCD detectors", Conference on Photoelectronic Image Devices, London, September, 1991, Section 3. Neither film nor SP imaging plates provide x-ray analysis in real time; they must first be removed from their apparatus before development or read out. Because of either their low stopping power or low x-ray quantum efficiency and long readout time, film and SP imaging plates perform poorly as synchrotron radiation detectors.
Some real time digitizing detectors for x-rays are known. Charge Coupled Device (CCD)-based detectors are superior to film and storage phosphor imaging plates in performance and convenience; however, they are limited by low signal-to-noise ratio, poor read out efficiency, limited effective temperatures of operation and poor time resolution compared to the present invention. Large-area, real-time, CCD-based digitizing detectors have been tested. See M. G. Strauss, et al., "CCD-based detector for protein crystallography with synchrotron X-rays", Nuclear Instruments and Methods in Physics Research A297 275-295, North Holland (1990). Such detectors use (1) a phosphor screen to detect the x-rays and convert one x-ray photon to many visible photons; (2) a fiber-optic and/or lens demagnifier to couple the phosphor screen to a charged coupled device (CCD) so that an area larger than the CCD can be used to collect x-rays (3) an image intensifier to increase the detector gain by further increasing the number of visible photons for each x-ray; (4) a CCD which detects the visible photons converting them to an electrical signal; (5) a cryostat to cool the CCD and reduce its noise. The CCD also integrates the signal and reads the signal out for digitization.
More recently modular digitizing crystallography detectors have been proposed for synchrotron-source studies. See FIG. 1 of S. M. Gruner et al, "Characterization of Polycrystalline Phosphors for Area X-ray Detectors", (July 1993), Proceedings of SPIE. These detectors do not include the image intensifier or lens demagnifiers; the fiber optics demagnifier is coupled directly to the CCD.
Large area detectors are desirable because they can collect a larger part of the crystallographic data; a high data redundancy is often advantageous in crystal analysis. Higher resolution requirements also lead to larger detector area requirements. Large detector area needs combined with small CCD size has necessitated the use of a fiber optic demagnifier and, in some detector designs, a lens demagnifier as well. This has led to a major shortcoming. Loss of more than 95% of the visible photons in making a direct transition from the phosphor to the CCD is common, resulting in low gain and, consequently, in a single-x-ray-photon signal which is well below the sensor noise and cannot be detected. The Detective Quantum Efficiency (DQE), and consequently the signal-to-noise ratio (S/N), for the collection of a few x-ray photons is very low. ((S/N).sup.2 =DQE.times.N, where N is the number of x-ray photons detected.) Attempts to design around this problem have resulted in the current CCD-based sensor designs.
(1) Where low flux signals are anticipated an image intensifier is required between the fiberoptic and the CCD to increase gain, increasing the signal above the noise. A weak link in low-signal detector designs is the image intensifier which, because of the low quantum efficiency of its photocathode or its large noise factor, results in a less than optimal DQE (about 50%-75%), and also causes inconvenient image distortion. Removing the image intensifier and directly coupling the fiber optics to the CCD (directly coupled detectors) requires a high-flux source to realize the benefits.
(2) Where high-flux synchrotron sources are available, the DQE, and thus the S/N, is increased by waiting to collect enough signal so that the signal dominates the internal noise. Intense sources will be effective in generating a high DQE, for signals of concern, as long as the detector meets certain minimum gain requirements and has a low internal noise. But the waiting times are excessive and preclude effective real time use. In addition, faint diffraction patterns may be undetected.
The CCD itself creates inherent time-resolution problems with the CCD-based detector designs resulting in considerable dead time when no light is being collected. Imaging CCD arrays are essentially slow readout devices with practical dynamic ranges of a few thousand. This makes them poor choices for dynamic, Laue-diffraction crystallographic studies or for optimally-efficient data collection with synchrotron sources. When data is collected, a shutter allows light to fall on the CCD and visible-photon signals are integrated. The shutter turns the light off and terminates the integration period. Data is then sequentially read off the CCD array. The pixels cannot be reset individually for continuous data collection so a considerable dead time exists. Readout time recommended for the Tektronix 1024.times.1024, 25 m pixel array is 20 .mu.s/pixel; the maximum readout rate for the front-illuminated version of this CCD is about 1 .mu.s/pixel. See Tektronix, "The Imaging CCD Array: Introduction and Operating Information; TK1024 CCD Imager", Technical note and Specifications, 1992 and 1991. However, increasing the readout speed above the recommended value results in increased noise. CCD-based detectors could, in principle, be used to obtain full-frame images with time resolution on the order of seconds. This is too slow for most high-flux sources and for most time resolved studies.
Alternatively, for time resolution studies, CCD-based detectors can be configured to collect a limited number of smaller-frame images: a restricted field-of-view causes phosphor light to fall on a small region of the CCD; pixels in this region are continuously clocked out of the light until all the pixels have been used. Thus, a number of small-frame images are stored and can be read out in the usual way. The problem with this method is the limited number of frames that can be stored and the limited number of pixels in a frame. The phosphor decay limits the use of the small-frame CCD method to a time resolution of about 1-10 ms. The CCD could be used to directly absorb the x-rays, but its area would be limited, due to the small CCD pixel size (about 25 .mu.m). The limited detector lifetime, using this non-radiation-hard technology, is a very important issue. Additionally, since the CCD has only a thin active region with not much x-ray stopping power, the DQE would be low, particularly for higher energy x-rays. We note that low sensitivity, or low DQE for a small number of x-ray photons, would limit the use of directly-coupled CCD detectors in time-resolved crystallography.
An SP-based system to record time-resolved x-ray patterns is depicted in Amemiya, Y. et al, (1989), Review of Scientific Instruments 60, 7, 1552-6. Like film in a movie camera, storage phosphors are mechanically continually replaced. It takes about 0.2 seconds to replace an SP and the minimum exposure time is about 0.1 second via a mechanical x-ray shutter; thus the time resolution is about 0.3 seconds. Because this is a mechanical system it is severely time-resolution limited.
Another mechanical device for obtaining time-resolved crystallographic images is a streak camera. See "Macromolecular Crystallography with Synchrotron Radiation", Helliwell, 1992, Cambridge, which discusses a rotating drum device in which x-rays, passing through a field-of-view-limiting slit, fall on an image plane (IP) located on the drum. Time evolution of a crystal, in a narrow field-of-view, is continually recorded on different areas of the SP with a maximum time-resolution of 0.023 ms. The limitations of this system are the one dimensionality of the image and the relatively low time resolution.
Another imaging detector-array technology under development is amorphous silicon. This technology is not suitable for dynamic-Laue studies because it is used with phosphor screens (the detectors have little x-ray stopping power), because it is a monolithic-design (small area available for circuitry) and because thin-film transistor (TFT) readout technology that is employed is too slow and unsophisticated (switching a signal onto an output line is about all that can be expected).